In the molding of plastic articles by injection molding machines, there is the constant problem of obtaining molded products at the lowest practical cost consistent with the quality of product desired. Obviously, the lower processing cost per item permits plastic molders to become more competitive and permits the use of molded plastic articles in an increasing number of products.
In producing plastic articles, resins are the basic ingredient used and represent the initial cost for a molded article. In order to reduce the resin cost, an entire industry has developed in supplying compounds of materials in which lower cost additives are mixed with a resin. Various compounding materials are used with resins to provide an effective way of tailoring moldable materials to obtain specific cost/performance needs for a molded article. The suppliers of such molding compounds are generally referred to as compounders and various processes for compounding resins and additives are well known to obtain the various commercial molding compounds. Although variations in compounding processes are known, compounding basically involves the introduction of a selected amount of a resin and one or more additives into a mixer to thereafter produce a homogeneous mixture of the resin and additive or additives. The homogeneous mixture is then supplied to a compounding extruder in which the mixture is heated to produce a plastic mass which heated mass is subsequently forced under high pressure through a die to produce continuous strands of compounded material. As the strands are discharged through the die, they enter a cooler, normally a water bath cooler, and are cooled to provide a solidified compounded plastic material. Thereafter the compounded material enters a pelletizer to produce pellets of a size as are desired by injection molders. The pellets are normally sized by being passed through a sizing screen and thereafter the sized pellets are mixed to obtain as homogeneous a mixture of pellets as is reasonable. When the pellets are received for processing by a molder they are first stored and, as desired, transported or conveyed to a material supply system of an injection molding machine. Frequently, due to variations in atmospheric conditions during the transportation and storage of the pellets, it is necessary to dry the pellets prior to their being conveyed to the material supply system so that the pellets have a sufficiently low moisture content to permit their subsequent use in an injection molding machine. The material supply systems frequently have auxiliary equipment for introducing still other additives to the pellets which are thoroughly mixed together prior to the entire mixture being discharged into the feed hopper of an injection molding machine. The entire mixture is fed from the feed hopper into the feed screw of an injection molding machine in a known manner and the entire mixture including the pellets are again heated into a molten plastic mass for subsequent forming into plastic articles. Obviously the cost of compounding and the handling of compounded pellets represent a substantial cost to a molder.